The present disclosure generally relates to operating mode detection in a flyback converter.
Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
FIG. 1 depicts a circuit 100 for a conventional flyback converter. In operation, a switch, such as a metal oxide field effect transistor (MOSFET) 102, may be turned on to connect a primary coil (an inductor 104a) to an input voltage VDC. In this case, a flyback transformer 106 is charged. A power factor correction (PFC) block 108 may also be included to provide power factor correction. PFC block 108 may turn MOSFET 102 on and off.
When MOSFET 102 is on, flyback transformer 106 causes a positive voltage at Vsec− with respect to Vsec+. This and a positive VOUT voltage cause diode 110 to be reverse biased or blocked. In this case, a capacitor 112 supplies energy to a load at VOUT.
When MOSFET 102 is turned off, the lack of current path at the primary side causes VPRI− to be charged to a voltage larger than VDC. This causes Vsec− to flyback to a negative voltage, which turns on diode 110. The energy of flyback transformer 106 is transferred to the output of the flyback converter. That is, current from an inductor 104b flows to the load.
The voltage drop across diode 110 is around 0.6 volts. This voltage drop results in a loss of efficiency in circuit 100. A MOSFET may be included in parallel with diode 110 to reduce the efficiency loss.
FIG. 1B depicts a circuit 113 of another example of a conventional flyback converter. A MOSFET 114 is included in parallel with diode 110. Also, a synchronous rectifier controller 116 is included and turns on MOSFET 114 when diode 110 is conducting, which reduces the current and voltage across diode 110. If the voltage drops below forward bias threshold of diode 110, all current flows through MOSFET 114.
In some applications, the flyback converter may operate in both continuous current mode (CCM) and discontinuous current mode (DCM). Therefore, synchronous rectifier controller 116 needs to function in both modes. However, both of these modes have different conditions to satisfy as to when synchronous rectifier controller 116 should turned off MOSFET 114. Thus, synchronous rectifier controller 116 needs to detect which mode the flyback converter is operating in to determine how to operate.
As discussed above, there are different conditions to satisfy as to when MOSFET 114 should be turned off for both of the modes. For example, in the continuous current mode, MOSFET 114 should be off before MOSFET 102 is turned on to prevent any short circuit current. In the discontinuous current mode, MOSFET 114 should be turned off before the current changes direction to prevent draining the charge of capacitor 112.
Synchronous rectifier controller 116 may be designed with two separate circuits to detect the two modes of operation. Also, one circuit may be used to detect both modes of operation. However, even if one mode is detected, synchronous rectifier controller 116 needs to make sure those conditions for the other mode are not met. This complicates the design because the logic cannot simply assume that if one mode is detected, this is the actual mode being used. A test whether the other mode is not in fact the operating mode is also performed.
In one solution, a user may decide in advance which mode of operation the flyback converter will be operating in by adjusting an inductance value of flyback transformer 106 and choosing the correct conditions to use to turn MOSFET 114 on and off. However, the mode may change with the load and may not be pre-determined beforehand. Thus, synchronous rectifier controller 116 still needs to continuously detect for any change in operating mode.